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Hello,
The low pressure port for charging the A/C system is located on the larger aluminum tube coming from the A/C compressor. It is usually located in the tube line near the accumulator. That is an alumninum cylinder about the size of a soda can or slightly larger usually located close to the firewall along the left fender side well. It will have a plastic screw on cap on it, usually stamped with an "L" on top. Your recharge hose will only connect to the low pressure side, the high pressure port is a different size valve for safety reasons. You can only get an accurate pressure reading with the engine running and A/C running at max cold. blower on high speed, after about 5 minutes of A/C operation. Temperature outside will also have an effect on the pressure reading, optimum ambient air temperature of 70-80 degrees farenheit. Directions for recharge are on your R134a refill kit, but the system does require 20-30 PSI for the compressor to kick on. If your compressor is engaging while you are refilling, you can probably do it successfully. If your pressure is extremely low and compressor does not kick on, you most likely have a leak in the system which will have to be found and fixed before you can charge the system......Mike

There is two filling ports for A/C - high pressure is biggest diameter-low pressure smallest one.All of them must have black cups.Because I don.t know exactly the model of your VW I can.t dive you proper instruction.If any problem come back with full details for the vehicle.

Did you check the acumulator? There is a port on it, that is the low side port. Keep in mind that if you have to recharge the system it is because other parts need service (leak in the syst) and whatever refrigerant you put in it will leak out again. The system may work temporarily or may work inefficiently. Also may sure that there is oil in syst.

there are 2 ports one for low pressure and one for high pressure. the low pressure is bigger in diameter and is usually close to the dryer and or fender and firewall. the high pressure is near the condensor on the fron of the vehicle. if there are caps for the ports low will be blue and high will be red. low side pressure should be 35-50 psi and high pressure should be 135-150 psi

There are two A/C refrigerant lines. The small line is your high pressure line and the larger line is your low pressure line.
Start at the fire wall and follow the "large " low pressure line to the compressor. The low pressure charge port will be some where on that line.
DO NOT try to recharge on the high pressure side ( small line )
Good luck, let me know.
Regards,